1. Field of the Invention
The present invention relates to a filter for use in radio apparatus, the filter comprising a plurality of resonators and plurality of variable capacitance means for electronically tuning respective resonators. The invention also relates to a radio apparatus comprising such a filter.
2. Description of the Related Art
Such a filter is known and is used in the front end of the Philips FM1000 mobile radio receiver. This filter comprises a plurality of resonators of the parallel LC type wherein the coupling between adjacent resonators is achieved by capacitive circuit elements rather than by electromagnetic coupling between the resonators themselves. These resonators have a high impedance and the capacitance of the variable capacitance means need only be a relatively small value to tune the resonators which does not seriously affect the performance of this filter. However these resonators have the disadvantage of being large and do not lend themselves to mass production. Furthermore the resonators in this filter have the further disadvantage of requiring shields and also requiring an isolated board for the high voltage end of the resonators.
Front end filters are of two general types, namely broad band fixed frequency filters and narrow band tunable filters which can be mechanically or electronically tuned the latter of which the FM1000 front end filter is an example. These narrow band tunable filters are more suitable for use in areas where there are a large number of transmitters and receivers and interference is a problem. In recent years it has been known to use bonded pairs or triplets of ceramic resonators in broad band fixed frequency filters where the coupling between the ceramic resonators is obtained purely by electromagnetic coupling and not by circuit elements. A filter is proposed consisting of a plurality of isolated ceramic resonators coupled via externally connected circuit elements. Isolated ceramic resonators have a low impedance and require a relatively large capacitive loading for electronic tuning. When these resonators are configured in the proposed filter and are electronically tuned, the coupling between adjacent resonators becomes difficult to control, in that the coupling co-efficient varies over the tuning range and the losses introduced by the tuning capacitance degrades the effective resonator Q.